Phosphorescent material, their preparations and applications

ABSTRACT

The subject invention is directed to tetradentate bis-(NHC carbenes) alkylene ligand Pt(II) complexes, tetradentate bis-(NHC carbenes) alkylene ligands, and its ligand precursors, for preparation of the Pt(II) complexes. The Pt(II) complexes show a deep blue emission with an improved quantum efficiency and can be used for fabrication of OLEDs with an electroluminescence layer that comprise the bis-(NHC carbenes) alkylene ligand Pt(II) complexes.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 61/491,711, filed May 31, 2011, which is hereby incorporated byreference in its entirety including any tables, figures, or drawings.

FIELD OF THE INVENTION

The invention relates to a class of phosphorescent materials, theirpreparation methods and organic light emitting diodes (OLEDs) usingthese materials.

BACKGROUND OF THE INVENTION

The first organic light-emitting diode (OLED) was disclosed by Tang et.al. (U.S. Pat. No. 4,356,429) and Tang et. al. (Appl. Phys. Lett. 1987,51, 12, 913). Subsequently, device architecture and emissive materialsfor OLED applications have been intensively studied. OLEDs allow devicesthat are (1) ultra-thin; (2) self-emissive; (3) use low operatingvoltage with high efficiency; and (4) display high contrast andresolution.

Phosphorescent materials are the primary direction of emissive materialdevelopment, because these OLED devices generate 25% singlet and 75%triplet excitons. Devices fabricated from phosphorescent materialsgenerally display efficiencies that are higher than devices fabricatedfrom fluorescent materials. Platinum complexes are a class of emissivematerials that offer high emission quantum efficiency and good thermalstability. High performance OLEDs have been formed using platinum(II)complexes. (Yan et al., Appl. Phys. Lett. 2007, 91(6) 063508; Che etal., Chemistry—A European Journal 2010, 16(1), 233)

Producing blue-emitting phosphorescent materials has proven to bechallenging. However, high performance and long lifetime blue-emittingOLEDs have not been fabricated from platinum(II) complexes. To tune theemission color of platinum(II) complexes, the appropriate conjugationlength of the ligand is essential. Neutral blue-emitting Pt(II) complexwere prepared by coordinating two bidentate ligands to a Pt(II) center,resulting in the first blue-emitting Pt(II) complexes. (Brooks et al.,Inorg. Chem. 2002, 41, 3055; and Unger et al., Angew. Chem. Int. Ed.2010, 49, 10214) However, binding forces between these bidentate ligandsand the platinum(II) center in these complexes are weaker than thebinding forces of complexes containing one tetradentate ligand bound toa Pt(II). The devices that form these bis-bidentate ligand Pt(II)complexes display lifetimes and stabilities that are inferior to devicetetradentate Pt(II) complexes. Ligand systems having more than two arylgroups, coupled by a non-conjugated unit, do not provide emission maximaless than 480 nm. (U.S. Pat. No. 7,026,480, U.S. Pat. No. 6,653,654).Blue-emitting materials have not been made from this type of tridentateor tetradentate ligands. Efforts directed to tetradentate blue-emittingPt(II) materials where conjugation groups connect aryl groups of theligands, have failed to generate complexes with emission maxima lessthan 480 nm. (U.S. Pat. No. 7,361,415, U.S. Pat. No. 7,691,495 and U.S.Published Patent Application 2007/0103060 A1)

N-heterocyclic carbenes (NHC) are strong σ-donors but poor π-acceptors.Using cyclometalated Pt(II) complexes, Meyer et al., Organometallics,2011, 30 (11), 2980 discloses Pt(II) complexes with shortened ligandπ-conjugations, but does not report of the emission spectra.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention are directed to blue phosphorescentplatinum(II) complexes of dianionic tetradentate bis-(NHC carbene)ligand of structure II:

wherein R₁-R₈ are, independently, hydrogen, fluoro, chloro, bromo, iodo,hydroxyl, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl,aryl, substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino,aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl,aryloxycarbonyl, phenoxycarbonyl, or alkoxycarbonyl, and X is,independently, oxygen, nitrogen, sulphur, phosphorus, or selenium. Otherembodiments of the invention are directed to the preparation of thetetradentate bis-(NHC carbene) ligands where at least one of R₁-R₈ arenot hydrogen, and OLEDs comprise Pt(II) complexes of tetradentatebis-(NHC carbene) ligands.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a reaction scheme for the preparation of tetradentatebis-(NHC carbene) ligand precursors, according to an embodiment of theinvention.

FIG. 2 shows the structure of three exemplary tetradentate bis-(NHCcarbene) ligand precursors, 1-3, according to an embodiment of theinvention.

FIG. 3 shows a reaction scheme for the conversion of a tetradentatebis-(NHC carbene) ligand precursor to a tetradentate bis-(NHC carbene)ligand Pt(II) complex, according to an embodiment of the invention.

FIG. 4 shows the structure of three exemplary tetradentate bis-(NHCcarbene) ligand Pt(II) complexes, 4-6, according to an embodiment of theinvention.

FIG. 5 shows the X-ray crystal structure of complex 5, according to anembodiment of the invention: a) as a perspective view with thermalellipsoids in 50% probability and hydrogen atoms and solvated moleculesomitted for clarity; and b) the packing diagram viewed along c-axis.

FIG. 6 shows absorption and emission spectra of 4, with absorption below382 and emission above 382, for a dichloromethane-DMF solution(19:1,v/v), THF-DMF solution (19:1,v/v) (blue shifted), and a film of 4in poly(methylmethacrylate) (1 wt %) (emission only and further blueshifted), according to an embodiment of the invention.

FIG. 7 shows absorption and emission spectra of 5, with absorption below382 and emission above 382, for a dichloromethane-DMF solution(19:1,v/v), THF-DMF solution (19:1,v/v) (blue shifted), and a film of 5in poly(methylmethacrylate) (1 wt %) (emission only and further blueshifted), according to an embodiment of the invention.

FIG. 8 shows absorption and emission spectra of 6, with absorption below382 and emission above 382, for a THF-DMF solution (19:1,v/v) (blueshifted), and a film of 6 in poly(methylmethacrylate) (1 wt %) (emissiononly and blue shifted), according to an embodiment of the invention.

FIG. 9 shows thermal gravimetric plots for complexes 4-7, according toan embodiment of the invention.

FIG. 10 shows the chemical structure of NPB, 2-TNATA, and TPBi, whichare used for the construction of an exemplary OLED with an emissivelayer comprising 5, according to an embodiment of the invention.

FIG. 11 shows an electroluminescence spectrum for the exemplary OLEDwith an emissive layer comprising 5, according to an embodiment of theinvention.

FIG. 12 shows a JVB cure for the exemplary OLED with an emissive layercomprising 5, according to an embodiment of the invention.

DETAILED DISCLOSURE OF THE INVENTION

Embodiments of the invention are directed to tetradentate ligandscomprising a bis-(NHC carbenes) alkylene, as shown in thebis-anion-bis-carbene form in structure I:

wherein R₁-R₈ are, independently, hydrogen, fluoro, chloro, bromo, iodo,hydroxyl, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl,aryl, substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino,aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl,aryloxycarbonyl, phenoxycarbonyl, or alkoxycarbonyl; and wherein atleast one of R₁-R₈ is not hydrogen; and X is O, NR₉, S, PR₉, or Se,where R₉ is H or alky. In an embodiment of the invention, R₆ is nothydrogen. In an embodiment of the invention, R₈ is not hydrogen.Embodiments of the invention are directed to stable blue-emittingplatinum(II) complexes prepared from ligands comprising bis-(NHCcarbenes) where the tetradentate ligand I's electron donor substituents,X, are in anions in a Pt complex of structure II:

wherein R₁-R₇ are, independently, hydrogen, fluoro, chloro, bromo, iodo,hydroxyl, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl,aryl, substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino,aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl,aryloxycarbonyl, phenoxycarbonyl, or alkoxycarbonyl; wherein at leastone of R₁-R₈ is not hydrogen; and X is O, NR₉, S, PR₉, or Se, where R₉is H or alky. In an embodiment of the invention, R₆ is not hydrogen. Inan embodiment of the invention, R₈ is not hydrogen

In an embodiment of the invention, the tetradentate ligand precursor isin the protonated form:

wherein R₁-R₇ are, independently, hydrogen, fluoro, chloro, bromo, iodo,hydroxyl, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl,aryl, substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino,aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl,aryloxycarbonyl, phenoxycarbonyl, or alkoxycarbonyl; wherein at leastone of R₁-R₈ is not hydrogen; X is O, NR₉, S, PR₉, or Se, where R₉ is Hor alky; and A⁻ is chloride, bromide, iodide, tosylate, brosylate,triflate, or other anion of low nucleophilicity. In an embodiment of theinvention, R₆ is not hydrogen. In an embodiment of the invention, R₈ isnot hydrogen.

Tetradentate ligand precursors III can be prepared by a series ofreactions depicted in FIG. 1, where A⁻ is Br⁻. Synthesis occurs throughreaction of an X protected o-halo substituted phenol, thiol, selenol,aniline, N-alkylaniline, phenylphosphane, or P-alkyl phenylphosphane,with imidazole, either unsubstituted, 4-substituted, or4,5-disubstituted. The resulting N-aryl substituted imidazolesdi-substitute a gem-dihaloalkane, or its functional equivalent towardsnucleophilic substitution, as can be appreciated by one skilled in theart. The protected precursor is one where H of the XH is replaced with aprotecting group that can be replaced with hydrogen, for example amethyl group. The reaction results in an X protected bis-(NHC carbenes)alkylene precursor, where upon deprotection, the bis-(NHC carbenes)alkylene ligand is provided as a precursor in a tetra-protonated form.Exemplary tetra-protonated bis-(NHC carbenes) alkylene ligandprecursors, according to an embodiment of the invention, are shown inFIG. 2.

Exemplary tetradentate bis-(NHC carbenes) alkylene ligand Pt(II)complexes, according to an embodiment of the invention, are shown inFIG. 4. In an embodiment of the invention, the tetradentate bis-(NHCcarbenes) alkylene ligand Pt(II) complex displays a vibronicallystructured absorption band, with a peak maximum at about 350 nm andmolar extinction coefficient of about 1×10⁴ M⁻¹ cm⁻¹, and displays, insolution, a structure-less emission having λ_(max) at about 460 nm witha quantum yield, (I), in excess of 5% and a lifetime, τ, in excess of1.5 μs. In embodiments of the invention, the blue emission of thebis-(NHC carbenes) alkylene ligand Pt(II) complex has a blue emission indilute solution with a λ_(max) less than 470 nm and a quantum yieldgreater than 5 percent. The bis-(NHC carbenes) alkylene ligand Pt(II)complex is a neutral square-planar Pt(II) complex, that is rigid andthermal stabile, wherein thermal decomposition occurs at temperaturesabove 280° C.

In an embodiment of the invention, the tetra-protonated bis-(NHCcarbenes) alkylene ligand precursor is combined with a Pt(II) salt toform a tetradentate bis-(NHC carbenes) alkylene ligand Pt(II) complex,as illustrated in FIG. 3. The Pt(II) salt can be a solvated orunsolvated platinum halide or its equivalent. For example thedimethylsulfoxide solvated PtCl₂, Pt(DMSO)₂Cl₂ can be used. Thecomplexation is carried out in solution in the presence of a protonacceptor, such as an amine. In an embodiment of the invention the protonacceptor is a tertiary amine, such as, but not limited to,trimethylamine, triethylamine, pyridine,N,N,N,N-tetramethylethylenediamine, or 1,4-dimethylpiperazine.

According to an embodiment of the invention, tetradentate bis-(NHCcarbenes) alkylene ligand Pt(II) complexes are dispersed and immobilizedin an inert polymer matrix, for example, but not limited to, poly(methylmethacrylate) (PMMA), to form a highly emissive film in the bluespectral region at a complex to polymer weight ratio of 1% or more.Absolute emission quantum yields for these films are around 30%, asmeasured with an integrate sphere method at room temperature. Emissionmaxima of the films can be blue-shifted by up to 10 nm from that for thebis-(NHC carbenes) alkylene ligand Pt(II) complexes in solution,suggesting a solid-solution state in the polymer dispersions. The filmsexhibit emission with chromaticity, as indicated by CommissionInternationale de L′Eclairage coordinates, of CIE_(x,y)<0.2 andCIE_(x+y)<0.3. For example, complex 4, shown in FIG. 4, displaysCIE_(x,y) of (0.15, 0.10), which is nearly that considered to be anideal deep blue, where CIE_(x,y) of (0.14, 0.10).

All patents and publications referred to or cited herein areincorporated by reference in their entirety, including all figures andtables, to the extent they are not inconsistent with the explicitteachings of this specification.

Following are examples that illustrate procedures for practicing theinvention. These examples should not be construed as limiting. Allpercentages are by weight and all solvent mixture proportions are byvolume unless otherwise noted.

Materials and Methods

All starting materials were used as received from commercial sources.The solvents used for photophysical measurements were of HPLC grade.Elemental analyses were performed by the Institute of Chemistry at theChinese Academy of Sciences, Beijing. Fast atom bombardment (FAB) massspectra were obtained on a Finnigan Mat 95 mass spectrometer. ¹H (300MHz or 400 MHz) NMR spectra were recorded on DPX300 and Avance400 BrukerFT-NMR spectrometers. UV-Vis spectra were recorded on a Perkin-ElmerLambda 19 UV/vis spectrophotometer. Steady-state emission and excitationspectra at 298 K and Photoluminescence of films on quartz substrate wereobtained on a Spex 1681 Fluorolog-2 Model F111 spectrophotometerequipped with a Hamamatsu R928PMT detector. All solutions forphotophysical measurements, except stated otherwise, were degassed in ahigh-vacuum line with at least four freeze-pump-thaw cycles. Emissionlifetimes were measured with a Quanta-Ray Q-switch DCR-3 Nd:YAG pulsedlaser system. Emission quantum yields were measured using a degassedacetonitrile solution of [Ru(bpy)₃](PF₆)₂ (bpy=2,2′-bipyridine) as thestandard (φ_(r)=0.062) and calculated byφ_(s)=φ_(r)(B_(r)/B_(s))(n_(s)/n_(r))²(D_(s)/D_(r)), where thesubscripts s and r refer to the sample and reference standard solution,respectively, n is the refractive index of the solvents, D is theintegrated intensity, and φ is the luminescence quantum yield. Thequantity B is calculated by B=1−10^(−AL), where A is the absorbance atthe excitation wavelength and L is the optical path length. Errors forwavelength values (1 nm) and φ (10%) are estimated.

4-Methyl-2-bromoanisole and 4-(tert-butyl)-2-bromoanisole were preparedfrom 4-Methyl-2-bromophenol and 4-(tert-butyl)-2-bromophenol by reactionwith MeI in the presence of K₂CO₃ following literature procedure. (Lygo,Tetrahedron Lett., 1999, 40, 1389)

4-methyl-2-bromoanisole

84% ¹H-NMR (400 MHz, CDCl₃) δ (ppm) 7.36 (d, J=2.1 Hz, 1H), 7.05 (dd,J=8.3 Hz, 2.1 Hz, 1H), 6.79 (d, J=8.3 Hz, 1H), 3.86 (s, 3H), 2.28 (s,3H)

4-(tert-butyl)-2-bromoanisole

¹H-NMR (300 MHz, CDCl₃) δ(ppm) 7.54 (d, J=2.2 Hz, 1H), 7.27 (dd, J=8.6Hz, 2.2 Hz, 1H), 6.83 (d, J=8.6 Hz, 1H), 3.87 (s, 3H), 1.28 (s, 9H).

2-(1H-imidazol-1-yl)anisole

To a degassed DMSO (20 mL) solution of imidazole (1.0 g, 15 mmol) wasadded 2-bromoanisole (1.25 mL, 10 mmol), KOH (1.12 g, 20 mmol) and Cu₂O(280 mg, 2 mmol) under nitrogen. The resulting mixture was stirred at140° C. for 24 hrs under N₂. After cooling to room temperature, themixture was poured into ethylacetate (EA) (50 mL) and filtered. Thefiltrate was washed with water (50 mL×3) and dried over anhydrousmagnesium sulfate. After rotary evaporation, the crude oil was purifiedvia column chromatography on silica gel with eluent of EA/MeOH (9/1,v/v), affording a light yellow liquid (50%). ¹H-NMR (400 MHz, CDCl₃)δ(ppm) 7.79 (s, 1H), 7.36 (t, J=7.8 Hz, 1H), 7.28 (d, J=7.8 Hz, 1H),7.21 (s, 1H), 7.17 (s, 1H), 7.05 (m, 2H), 3.85 (s, 3H).

4-fluoro-2-(1H-imidazol-1-yl)anisole

To a degassed DMSO (20 mL) solution of imidazole (1.0 g, 15 mmol) wasadded 4-fluoro-2-bromoanisole (1.25 mL, 10 mmol), KOH (1.12 g, 20 mmol)and Cu₂O (280 mg, 2 mmol) under nitrogen. The resulting mixture wasstirred at 140° C. for 24 hrs under N₂. After cooling to roomtemperature, the mixture was poured into ethylacetate (EA) (50 mL) andfiltered. The filtrate was washed with water (50 mL×3) and dried overanhydrous magnesium sulfate. After rotary evaporation, the crude oil waspurified via column chromatography on silica gel with eluent of EA/MeOH(9/1, v/v), affording a light yellow liquid (47%). ¹H-NMR (400 MHz,CDCl₃) δ(ppm) 7.82 (s, 1H), 7.21 (m, 1H), 7.17 (m, 1H), 7.07 (m, 2H),7.00 (m, 1H), 3.84 (s, 3H).

4-methyl-2-(1H-imidazol-1-yl)anisole

To a degassed DMSO (20 mL) solution of imidazole (1.0 g, 15 mmol) wasadded 4-methyl-2-bromoanisole (1.25 mL, 10 mmol), KOH (1.12 g, 20 mmol)and Cu₂O (280 mg, 2 mmol) under nitrogen. The resulting mixture wasstirred at 140° C. for 24 hrs under N₂. After cooling to roomtemperature, the mixture was poured into ethylacetate (EA) (50 mL) andfiltered. The filtrate was washed with water (50 mL×3) and dried overanhydrous magnesium sulfate. After rotary evaporation, the crude oil waspurified via column chromatography on silica gel with eluent of EA/MeOH(9/1, v/v), affording a light yellow solid (57%). ¹H-NMR (300 MHz,CDCl₃) δ(ppm) 7.78 (s, 1H), 7.16 (m, 3H), 7.09 (d, J=1.8 Hz, 1H), 6.94(d, J=8.4 Hz, 1H), 3.81 (s, 3H), 2.33 (s, 3H).

4-t-butyl-2-(1H-imidazol-1-yl)anisole

To a degassed DMSO (20 mL) solution of imidazole (1.0 g, 15 mmol) wasadded 4-methyl-2-bromoanisole (1.25 mL, 10 mmol), KOH (1.12 g, 20 mmol)and Cu₂O (280 mg, 2 mmol) under nitrogen. The resulting mixture wasstirred at 140° C. for 24 hrs under N₂. After cooling to roomtemperature, the mixture was poured into ethylacetate (EA) (50 mL) andfiltered. The filtrate was washed with water (50 mL×3) and dried overanhydrous magnesium sulfate. After rotary evaporation, the crude oil waspurified via column chromatography on silica gel with eluent of EA/MeOH(9/1, v/v), affording a light yellow oil with yield of 47%. ¹H-NMR (400MHz, CDCl₃) δ(ppm) 7.77 (s, 1H), 7.37 (dd, J₁=8.6 Hz, J₂=2.4 Hz, 1H),7.27 (d, J=2.4 Hz, 1H), 7.21 (1H), 7.17 (1H), 6.99 (d, J=8.6 Hz, 1H),3.82 (s, 3H), 1.32 (s, 9H).

1,1′-bis[(2-methoxyphenyl)-1H-imidazolium]-3,3′-methanediyl dibromide

A solution of 2-(1H-imidazol-1-yl)anisole (0.82 g, 4.7 mmol) anddibromomethane (1 mL, 14 mmol) in THF (5 mL) was refluxed at 110° C. for48 hrs. After cooling to room temperature, the resulting whiteprecipitate was collected by filtration with suction, washed with THF,and air-dried to yield 0.9 g of solids (73%). ¹H-NMR (400 MHz, DMSO-d₆)δ(ppm) 10.01 (s, 2H), 8.30 (m, 2H), 8.22 (m, 2H), 7.64 (m, 4H), 7.41 (d,J=8.3 Hz, 2H), 7.22 (t, J=7.7 Hz, 2H), 6.90 (s, 2H), 3.90 (s, 6H).

1,1′-bis[(4-fluoro-2-methoxyphenyl)-1H-imidazolium]-3,3′-methanediyldibromide

A solution of 4-fluoro-2-(1H-imidazol-1-yl)anisole (0.90 g, 4.7 mmol)and dibromomethane (1 mL, 14 mmol) in THF (5 mL) was refluxed at 110° C.for 48 hrs. After cooling to room temperature, the resulting whiteprecipitate was collected by filtration with suction, washed with THF,and air-dried to yield a white solid with yield of 70%. ¹H-NMR (400 MHz,MeOD) o(ppm) 8.33 (d, J=2.1 Hz, 2H), 8.11 (d, J=2.1 Hz, 2H), 7.66 (m,2H), 7.40 (m, 4H), 7.11 (s, 2H), 3.98 (s, 6H). Note: the imidazole NCHNsignal do not appeared in MeOD solvent.

1,1′-bis[(4-methyl-2-methoxyphenyl)-1H-imidazolium]-3,3′-methanediyldibromide

A solution of 4-methyl-2-(1H-imidazol-1-yl)anisole (0.88 g, 4.7 mmol)and dibromomethane (1 mL, 14 mmol) in THF (5 mL) was refluxed at 110° C.for 48 hrs. After cooling to room temperature, the resulting whiteprecipitate was collected by filtration with suction, washed with THF,and air-dried to yield a white solid with yield of 70%. ¹H-NMR (300 MHz,DMSO-d₆) δ(ppm) 9.93 (s, 2H), 8.26 (m, 2H), 8.20 (m, 2H), 7.47 (d, J=1.6Hz, 2H), 7.44 (dd, J₁=8.6 Hz, J₂=1.6 Hz, 2H), 7.31 (d, J=8.6 Hz, 2H),6.86 (s, 2H), 3.88 (s, 3H), 2.33 (s, 3H).

1,1′-bis[(4-t-butyl-2-methoxyphenyl)-1H-imidazolium]-3,3′-methanediyldibromide

A solution of 4-t-butyl-2-(1H-imidazol-1-yl)anisole (1.08 g, 4.7 mmol)and dibromomethane (1 mL, 14 mmol) in THF (5 mL) was refluxed at 110° C.for 48 hrs. After cooling to room temperature, the resulting whiteprecipitate was collected by filtration with suction, washed with THF,and air-dried to yield a white solid with yield of 74%. ¹H-NMR (400 MHz,DMSO-d₆) δ(ppm) 10.00 (s, 2H), 8.31 (m, 2H), 8.24 (m, 2H), 7.64 (m, 4H),7.33 (d, J=8.8 Hz, 2H), 6.89 (s, 2H), 3.89 (s, 3H), 1.28 (s, 18H).

1,1′-bis[(2-hydroxyphenyl)-1H-imidazolium]-3,3′-methanediyl dibromide

A solution of1,1′-bis[(2-methoxyphenyl)-1H-imidazolium]-3,3′-methanediyl dibromide(0.81 g, 1.55 mmol) in HBr (48 wt. % aq. 6.5 mL)/HOAc (6.5 mL) washeated to reflux at 120° C. for 48 hrs. After reaction, the mixture wasrotary evaporated. Acetone was added to the residue to induceprecipitation of brown solids, which were collected by filtration andwashed with EA. After reprecipitation from DMF/EA, a white solid wasobtained (0.52 g, 68%). ¹H-NMR (400 MHz, DMSO-d₆) δ(ppm) 11.08 (br s,2H), 9.99 (s, 2H), 8.29 (s, 2H), 8.22 (s, 2H), 7.57 (d, J=7.86 Hz, 2H),7.44 (t, J=7.77 Hz, 2H), 7.19 (d, J=8.13 Hz, 2H), 7.06 (t, J=7.58 Hz,2H), 6.90 (s, 2H).

1,1′-bis[(4-fluoro-2-hydroxyphenyl)-1H-imidazolium]-3,3% methanediyldibromide (1)

A solution of1,1′-bis[(4-fluoro-2-methoxyphenyl)-1H-imidazolium]-3,3′-methanediyldibromide (0.87 g, 1.55 mmol) in HBr (48 wt. % aq. 6.5 mL)/HOAc (6.5 mL)was heated to reflux at 120° C. for 48 hrs. After reaction, the mixturewas rotary evaporated. Acetone was added to the residue to induceprecipitation of brown solids, which were collected by filtration andwashed with EA. After reprecipitation from MeOH/EA, a white solid wasobtained with yield of 84% ¹H-NMR (300 MHz, MeOD) δ(ppm) 9.97 (im NCHN,s, active), 8.23 (d, J=2.0 Hz, 2H), 8.13 (d, J=2.0 Hz, 2H), 7.52 (dd,J₁=8.4 Hz, J₂=3.0 Hz, 2H), 7.26 (td, J₁=9.0 Hz, J₂=3.0 Hz, 2H), 7.14(dd, J₁=9.0 Hz, J₂=4.8 Hz, 2H), 7.00 (s, 2H).

1,1′-bis[(4-methyl-2-hydroxyphenyl)-1H-imidazolium]-3,3%-methanediyldibromide (2)

A solution of1,1′-bis[(4-methyl-2-methoxyphenyl)-1H-imidazolium]-3,3′-methanediyldibromide (0.85 g, 1.55 mmol) in HBr (48 wt. % aq. 6.5 mL)/HOAc (6.5 mL)was heated to reflux at 120° C. for 48 hrs. After reaction, the mixturewas rotary evaporated. Acetone was added to the residue to induceprecipitation of brown solids, which were collected by filtration andwashed with EA. After reprecipitation from MeOH/EA, a white solid wasobtained with yield of 65% ¹H-NMR (300 MHz, DMSO-d₆) δ(ppm) 10.82 (br s,2H), 9.96 (s, 2H), 8.28 (m, 2H), 8.20 (m, 2H), 7.39 (d, J=1.6 Hz, 2H),7.25 (dd, J₁=8.4 Hz, J₂=1.6 Hz, 2H), 7.08 (d, J=8.4 Hz, 2H), 6.88 (s,2H), 2.28 (s, 6H). (300 MHz, MeOD) δ(ppm) 9.91 (im NCHN, s, 2H), 8.20(m, 2H), 8.09 (m, 2H), 7.43 (d, J=1.6 Hz, 2H), 7.28 (dd, J₁=8.4 Hz,J₂=1.6 Hz, 2H), 7.03 (d, J=8.4 Hz, 2H), 6.98 (s, 2H), 2.35 (s, 6H).

1,1′-bis[(4-t-butyl-2-hydroxyphenyl)-1H-imidazolium]-3,3% methanediyldibromide (3)

A solution of1,1′-bis[(4-t-butyl-2-methoxyphenyl)-1H-imidazolium]-3,3′-methanediyldibromide (0.98 g, 1.55 mmol) in HBr (48 wt. % aq. 6.5 mL)/HOAc (6.5 mL)was heated to reflux at 120° C. for 48 hrs. After reaction, the mixturewas rotary evaporated. Acetone was added to the residue to induceprecipitation of brown solids, which were collected by filtration andwashed with EA. After reprecipitation from DMF/EA, a white solid wasobtained with yield of 62%. ¹H-NMR (400 MHz, DMSO) δ(ppm) 10.85 (s, 2H),9.91 (s, 2H), 8.24 (m, 4H), 7.48 (m, 4H), 7.11 (d, J=8.4 Hz), 6.84 (s,2H), 1.29 (s, 18H).

Bis[3,3′-(2-phenoxide)-1H-imidazolium-2,2′-diylidene]-methane-1,1′-diyl-platinum(H)(7)

A mixture of 1,1′-bis[(2-hydroxyphenyl)-1H-imidazolium]-3,3′-methanediyldibromide (193 mg, 0.39 mmol), Pt(DMSO)₂Cl₂ (164 mg, 0.39 mmol) and Et₃N(0.325 mL, 6 eq.) in EtOH (20 mL) was heated to 80° C. for 6 hrs. Aftercompletion of reaction, the mixture was cooled to room temperature,collected by centrifugation, washed with ethanol, ether and was driedunder vacuum, affording pale yellow solids (60%). ¹H-NMR (400 MHz,DMSO-d₆) δ(ppm) 8.34 (d, J=2.3 Hz, 2H), 7.71 (d, J=2.3 Hz, 2H), 7.69 (d,J=8.35 Hz, 2H), 7.06 (t, J=7.65 Hz, 2H), 6.92 (d, J=8.27 Hz, 2H), 6.57(t, J=7.57 Hz, 2H), 6.31 (s, 2H) FAB-MS: 526.0 [M+H]⁺. Anal. Calcd. ForC₁₉H₁₄N₄O₂Pt.H₂O: C, 41.99; H, 2.97; N, 10.31. Found: C, 42.06; H, 2.88;N, 9.99.

Bis[3,3′-(4-fluoro-2-phenoxide)-1,1-imidazolium-2,2′-diylidene]-methane-1,1′-diyl-platinum(II)(4)

A mixture of 1 (207 mg, 0.39 mmol), Pt(DMSO)₂Cl₂ (164 mg, 0.39 mmol) andEt₃N (0.325 mL, 6 eq.) in EtOH (20 mL) was heated to 80° C. for 6 hrs.After completion of reaction, the mixture was cooled to roomtemperature, collected by centrifugation, washed with ethanol, ether andwas dried under vacuum, affording white solids with yield of 49%. ¹H-NMR(400 MHz, DMSO-d₆) δ(ppm) 8.34 (d, J=1.8 Hz, 2H), 7.72 (d, J=1.8 Hz,2H), 7.66 (d, J=10.5 Hz, 2H), 6.90 (m, 4H), 6.32 (s, 2H) FAB-MS: 561.0[M+H]⁺. Anal. Calcd. For C₁₉H₁₂N₄O₂Pt.H₂O: C, 39.38; H, 2.44; N, 9.67.Found: C, 38.77; H, 2.40; N, 9.35.

Bis[3,3′-(4-methyl-2-phenoxide)-1H-imidazolium-2,2′-diylidene]-methane-1,1′-diyl-platinum(II)(5)

A mixture of 2 (204 mg, 0.39 mmol), Pt(DMSO)₂Cl₂ (164 mg, 0.39 mmol) andEt₃N (0.325 mL, 6 eq.) in EtOH (20 mL) was heated to 80° C. for 6 hrs.After completion of reaction, the mixture was cooled to roomtemperature, collected by centrifugation, washed with ethanol, ether andwas dried under vacuum, affording white solids with yield of 56%. ¹H-NMR(300 MHz, DMSO-d₆) δ(ppm) 8.31 (d, J=2.1 Hz, 2H), 7.69 (d, J=2.1 Hz,2H), 7.51 (s, 2H), 6.87 (d, J=8.2 Hz, 2H), 6.80 (d, J=8.2 Hz, 2H), 6.29(s, 2H), 2.25 (s, 6H) FAB-MS: 553.0 [M+H]⁺. Anal. Calcd. ForC₂₁H₁₈N₄O₂Pt.CH₂Cl₂: C, 41.39; H, 3.16; N, 8.78. Found: C, 41.38; H,3.22; N, 8.82.

A single-crystal of 5 suitable for X-ray diffraction analysis wasobtained from a dilute solution in dichloromethane upon slow evaporationof the solvent in the presence of air. The complex crystallized into anorthorhombic space group, as indicated in Table 1, below. As can be seenin FIG. 5 a, Complex 5 has a C₂-symmetry along the Pt1-C11 axis. Thefour bond angles around Pt(II) are within 90±2°, indicating asquare-planar coordination geometry with a negligible strain in thetetradentate ligand accommodating the Pt(II) ion. The Pt-C(NHC)distances of 1.93 Å are shorter than those found in tetra(NHC)-Pt(II)(2.03 Å) and bis(NHC)-Pt(II) acetylide (1.98 Å) complexes, indicatingstrong Pt-C(NHC) bonding interactions. The Pt—O(phenolate) distances of2.05 Å are slightly longer those found in Salphen-Pt(II) (1.99 Å) and(N₂O₂)-Pt(II) (1.98 Å) complexes, probably due to weak Pt.O(phenolate)bonds induced by the strong trans-effect of the NHC moieties. Themolecules of 5 adopt a bent framework due to the non-conjugated C11methylene linker between the two NHC moieties, and these bent moleculesstack into infinite columns in a head-to-tail cofacial manner withintermolecular π-π distances of 3.5 Å along a-axis, as shown in FIG. 5b. A solvated water molecule is hydrogen-bonded to the two phenolateoxygen atoms of a complex molecule. Disordered solvating dichloromethanemolecules are located in the channels between the columns formed by thestacked 5 molecules.

TABLE 1 Crystal Data of complex 3 formula C₂₂H₂₂Cl₂N₄O₃Pt fw 656.43color Colorless crystal size 0.1 × 0.04 × 0.02 crystal systemOrthorhombic space group Pnma a, Å 6.7618(2) b, Å 24.1817(7) c, Å13.4445(4) α, deg 90 β, deg 90 γ, deg 90 V, Å³ 2198.33 Z 4 D_(c), g cm⁻³1.983 μ, cm⁻¹ 14.461 F(000) 1272 2θ_(max), deg 130.81 no. unique data1872 no. obsd. data GOF 1.191 for I > 2σ (I) no. variables 155 R^(a)0.0517 Rw^(b) 0.0968 residual ρ, e Å⁻³ 1.364, −2.386 ^(a)R = Σ||F_(o)| −|F_(c)||/Σ|F_(o)|. ^(b)R_(w) = [Σ w (|F_(o)| − |F_(c)|)²/Σ w|F_(o)|²]^(1/2).

Bis[3,3′-(4-t-butyl-2-phenoxide)-1,1-imidazolium-2,2′-diylidene]-methane-1,1′-diyl-platinum(II)(6)

A mixture of 3 (237 mg, 0.39 mmol), Pt(DMSO)₂Cl₂ (164 mg, 0.39 mmol) andEt₃N (0.325 mL, 6 eq.) in EtOH (20 mL) was heated to 80° C. for 6 hrs.After completion of reaction, the mixture was cooled to roomtemperature, collected by centrifugation, washed with ethanol, ether andwas dried under vacuum, affording white solids with yield of 58%. ¹H-NMR(400 MHz, DMSO-d₆) δ(ppm) 8.42 (d, 2.3 Hz, 2H), 7.71 (d, 2.3 Hz, 2H),7.52 (d, J=2.3 Hz, 2H), 7.08 (dd, J₁=8.6 Hz, J₂=2.3 Hz, 2H), 6.83 (d,J=8.6 Hz, 2H), 6.30 (s, 2H), 1.29 (s, 18H) FAB-MS: 638.2 [M+H]⁺. Anal.Calcd. For C₂₇H₃₀N₄O₂Pt.H₂O: C, 49.46; H, 4.92; N, 8.55. Found: C,49.36; H, 4.64; N, 8.46.

Absorption and emission spectra were acquired for complexes 4-7, assummarized in Table 2, below, solution measurements were performed usingTHF-DMF (19:1, v/v) solutions. All of the complexes show a vibronicallystructured absorption band with peak maxima at about 350 nm and molarextinction coefficients of about 1×10⁴ M⁻¹ cm⁻¹. In solution, complexes5 and 6 display structure-less emissions centered at around 460 nm withquantum yields of 8% and lifetimes of 1.8 μs, which is significantlygreater than that for 7, the complex from an unsubstituted ligand. Ablue-shifted emission maximum at 443 nm with a high quantum yield of 18%and long lifetime of 3.5 μs was observed in solution for complex 4 withan electron-drawing fluoride group para to the phenolate oxygen. Ared-shift of 5 nm for the emission maxima for 4-7 was observed uponchanging solvent from THF-DMF to dichloromethane-DMF (19:1,v/v).Representative adsorption and emission spectra for 4-6 are shown inFIGS. 6-8 respectively.

Complexes 4-7 are highly emissive in the blue spectral region whenmeasured in films prepared by the dispersion of a complex in an inertpolymer matrix, poly(methyl methacrylate) (PMMA) in these exemplaryexamples, at a complex to polymer weight ratio as low as 1%. Absoluteemission quantum yields for these solid films, measured by an integratesphere method, were observed to be approximately 30%, withinexperimental error, at room temperature for 4-7 with emission maximablue-shifted by 10 nm from that recorded for the complexes in THF-DMFsolution, which suggests a solid-solution state within the film. All ofthe films exhibit emission with a chromaticity of CIE_(x,y)<0.2(CIE_(x,y)=Commission Internationale de L'Eclairage coordinates) andCIE_(x+y)<0.3, and particularly, films of complex 4 gave CIE_(x,y) at(0.15, 0.10), being close to an ideal deep blue with CIE_(x,y) at (0.14,0.10).

TABLE 2 Photophysical data of complexes 4-7. Absorption^([a]) λ_(max) innm Emission^([a]) Emission^([b]) (ε in M⁻¹cm⁻¹) λ_(max) (nm); τ (μs); φ(%) λ_(max)/nm; φ 4 286 (8.5), 310 (9.7), 443, 459; 3.5; 18% 434, 451;26% 353 (10.4), 365 (10.5) 5 284 (10.0), 308 (9.4), 460; 1.8; 7% 448;24% 351 (9.7), 362 (9.7) 6 282 (11.5), 308 (10.5), 461; 1.8; 8% 449; 26%352 (12.3), 363 (12.7) 7 277 (8.4), 302 (7.3), 457; 0.5; 3% 443; 29% 342(7.1), 353 (7.1) ^([a])Recorded in degassed THF-DMF (19:1, v/v)solutions with a concentration of ~2 × 10⁻⁵ M, absorption at about 305is a shoulder, and λ_(ex) for emissions is 350 nm. ^([b])Recorded in 1%PMMA film and λ_(ex) for emissions is 350 nm.

Decomposition temperatures of 4-6 under a nitrogen atmosphere are 410,390, and 400° C., respectively, as deteunined by thermogravimetricanalyses, as shown in FIG. 9. Complexes 4-6 meet the prerequisite forthermal deposition of these complexes in OLED fabrications and exceedsthat of unsubstituted complex 7 which decomposed beginning at 250° C.

Complex 6 was used as phosphorescent dopant in a blue OLED. The deviceconfiguration is ITO/2-TNATA (40 nm)/NPB (20 nm)/DP4-Pt 3% (30 nm)/TPBi(40 nm)/LiF (0.5 nm)/Al (100 nm). 2-TNATA, NPB and TPBi were used ashole-injection layer (HIL), hole-transporting layer (HTL) andelectron-transporting layer (ETL), respectively. The host DP4 wasobtained from Aglaia Tech. Beijing, China, and its chemical structurecannot be revealed due to patent consideration. All materials werethermal-deposited in high vacuum in succession without breaking thevacuum. After finishing the whole deposition, the device wasencapsulated with glass cap and tested at ambient conditions. Theelectroluminescence was recorded by a PR650-spectrometer and K2400 asthe voltage source.

An OLED was fabricated using 6, which was vacuum-deposited as a dopantinto the emitting layer of an OLED with a configuration of ITO/2-TNATA(40 nm)/NPB (20 nm)/DP4-Complex 102 (30 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al(100 nm). 2-TNATA, NPB and TPBi, whose structures are shown in FIG. 10,were used as hole-injection, hole-transporting and electron-transportinglayer, respectively. Complex 6 was co-deposited with a broad-gap host(DP4, provided by Aglaia Tech., Beijing, China) at a 3 wt % dopinglevel. The electroluminescence spectrum and JVB curve of the device areillustrated in FIGS. 11 and 12, respectively. A maximum brightness of1200 cd/m² was recorded at a driving voltage of 15 V. The chromaticityof this device at 11 V is CIE_(x,y) (0.16, 0.16), falling well into theblue spectral region. A peak luminous efficiency of 0.5 cd/A wasrecorded at 11 V.

It should be understood that the examples and embodiments describedherein are for illustrative purposes only and that various modificationsor changes in light thereof will be suggested to persons skilled in theart and are to be included within the spirit and purview of thisapplication and the scope of the appended claims. In addition, anyelements or limitations of any invention or embodiment thereof disclosedherein can be combined with any and/or all other elements or limitations(individually or in any combination) or any other invention orembodiment thereof disclosed herein, and all such combinations arecontemplated with the scope of the invention without limitation thereto.

1.-22. (canceled)
 23. A tetradentate ligand, comprising abis-anion-bis-(NHC carbene) alkylene of the structure:

wherein R₁-R₈ are independently hydrogen, fluoro, chloro, bromo, iodo,hydroxyl, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl,aryl, substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino,aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl,aryloxycarbonyl, phenoxycarbonyl, or alkoxycarbonyl; wherein at leastone of R₁-R₈ is not hydrogen; and wherein X is independently O, NR₉, S,PR₉, or Se, where R₉ is H or alky.
 24. The tetradentate ligand of claim23, wherein R₆ or R₈ is substituted with an electron withdrawingsubstituent.
 25. The tetradentate ligand of claim 23, wherein R₆ isfluoro.
 26. A precursor to the tetradentate ligand of claim 23,comprising the structure:

wherein R₁-R₈ are independently hydrogen, fluoro, chloro, bromo, iodo,hydroxyl, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl,aryl, substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino,aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl,aryloxycarbonyl, phenoxycarbonyl, or alkoxycarbonyl; wherein at leastone of R₁-R₈ is not hydrogen; wherein X is independently O, NR₉, S, PR₉,or Se, where R₉ is H or alky; and wherein A⁻ is chloride, bromide,iodide, tosylate, brosylate, triflate, or other anion of lownucleophilicity.
 27. The precursor of claim 26, wherein R₆ or R₈ issubstituted with an electron withdrawing substituent.
 28. The precursorof claim 26, wherein R₆ is fluoro.
 29. The precursor of claim 26,wherein A⁻ is bromide.
 30. The precursor of claim 26, wherein thestructure is:


31. The precursor of claim 26, wherein H of the XH is replaced with amethyl or other protecting group.
 32. A tetradentate bis-(NHC carbenes)alkylene ligand Pt(II) complex comprising the ligand of claim 1,comprising the structure:

wherein R₁-R₈ are independently hydrogen, fluoro, chloro, bromo, iodo,hydroxyl, alkyl, substituted alkyl, cycloalkyl, substituted cycloalkyl,aryl, substituted aryl, acyl, alkoxy, acyloxy, amino, nitro, acylamino,aralkyl, cyano, carboxyl, thio, styryl, aminocarbonyl, carbamoyl,aryloxycarbonyl, phenoxycarbonyl, or alkoxycarbonyl; wherein at leastone of R₁-R₈ is not hydrogen; and wherein X is independently O, NR₉, S,PR₉, or Se, where R₉ is H or alky.
 33. The tetradentate bis-(NHCcarbenes) alkylene ligand Pt(II) complex of claim 32, wherein R₆ or R₈is substituted with an electron withdrawing substituent.
 34. Thetetradentate bis-(NHC carbenes) alkylene ligand Pt(II) complex of claim32, wherein R₆ is fluoro.
 35. The tetradentate bis-(NHC carbenes)alkylene ligand Pt(II) complex of claim 32, wherein the tetradentateligand Pt(II) complex has a blue emission with a maximum less than 470nm.
 36. The tetradentate bis-(NHC carbenes) alkylene ligand Pt(II)complex of claim 35, wherein the tetradentate ligand Pt(II) complex hasthe blue emission with a quantum efficiency in solution of at least 5percent.
 37. The tetradentate bis-(NHC carbenes) alkylene ligand Pt(II)complex of claim 32, of the structure:


38. A method for the preparation of the tetradentate bis-(NHC carbenes)alkylene ligand Pt(II) complex of claim 11, comprising combining aprecursor of claim 32 with a Pt(II) salt in the presence of a protonacceptor in solution.
 39. The method of claim 38, wherein the protonacceptor is a tertiary amine.
 40. The method of claim 38, wherein thePt(II) salt is a Pt(II) halide.
 41. The method of claim 38, wherein thePt(II) halide is Pt(DMSO)₂Cl₂.
 42. An organic light-emitting diode(OLED), comprising an electroluminescence layer comprising atetradentate bis-(NHC carbenes) alkylene ligand Pt(II) complex of claim32.
 43. An organic light-emitting diode (OLED) of claim 42, wherein thetetradentate bis-(NHC carbenes) alkylene ligand Pt(II) complex has astructureless blue emission in dilute solution having a λ_(max) lessthan 470 nm with a quantum yield, φ, greater than 5%.